1. Field of the Invention
The present invention relates according to the preamble of claim 1.
2. Discussion of Background
It is normal practice to use compressed air for cooling thermally highly loaded units of a gas-turbine group, for example a combustion chamber, which compressed air, for this purpose, is branched off intermediately or after compression has been effected. However, it is now the case that, in modern gas turbines, the fuel should be premixed with if possible all the compressor air in order to achieve low-pollution combustion at maximized efficiency. Air losses, as occur in the known air-cooling systems of the thermally highly loaded units, are therefore to be avoided. A further problem in such air-cooling systems consists in the fact that the thermally consumed air in each case has to be fed back into the turbine process at a suitable point, in which case an air quantity, which has cooled the turbine for example, then cannot readily be directed as cooling medium into the combustion chamber, since higher pressures regularly prevail there. Cooling potential is therefore always lost during the feedback of this cooling air into the turbine process and has to be replaced by other media.
This initial situation comes to the fore, for example, if the gas-turbine group is designed for sequential firing.
In the case of gas turbines which are operated in combination with a downstream steam circuit, the practice of using steam, which is available anyway, for the cooling of the thermally highly loaded units has been adopted, based on the fact that the waste heat available from the last gas turbine is able to generate more steam than is necessary to operate the steam turbine belonging to the steam circuit. It has become known to introduce this freely available steam into the gas-turbine process in order to increase the output. In this case, for thermodynamic and fluidic reasons, attention is drawn to the fact that the admixing with the combustion air cannot impair the combustion as such. However, this regularly leads to complicated injection techniques, which not least are connected with the mixture formation of two media which are different in temperature, pressure and behavior. In addition, it must not be left unmentioned that the fuel control is extremely difficult to handle here.